Pressure swing adsorption is an important gas separation process which is widely used in the chemical process industries. The process has been highly developed in particular for use in the recovery of high purity hydrogen from synthesis gas, refinery offgases, and other hydrogen-containing gas mixtures. The process as known in the art uses multiple adsorbent beds operating in parallel with overlapping cycles to provide selective gas flow between beds for pressure equalization and purge steps. Highly developed cycles are known which use up to 16 beds with multiple beds on feed and multiple beds on purge at any given time. Each bed can undergo multiple pressure equalization steps with other individual beds in order to increase product gas recovery.
Each adsorbent bed in a pressure swing adsorption (PSA) cycle typically utilizes a sequence of steps which begins with a feed or adsorption step in which a pressurized feed gas mixture is passed through a bed of adsorbent which selectively adsorbs one or more of the components in the mixed feed gas. A product gas containing the desired component at acceptable purity is withdrawn from the bed until the adsorption step is terminated at a predetermined time.
After termination of the adsorption step, the pressure in the bed is reduced in a series of pressure equalization steps in which gas is transferred at decreasing pressure to a succession of other beds to provide pressurization to those beds. Final depressurization typically is completed by withdrawing a waste gas in a final blowdown step. The depressurized bed then is purged with purge gas provided from other beds, thereby removing additional adsorbed components from the bed.
Upon completion of the purge step, the bed is repressurized to an intermediate pressure by a succession of pressure equalization steps in which gas is transferred from other beds, and the bed is pressurized further to the feed pressure with feed and/or product gas. The steps are repeated in a cyclic manner.
An objective in the development of pressure swing adsorption cycles is to reduce the cycle time in order to reduce the amount of adsorbent required in the beds for a given feed rate while continuing to provide product at an acceptable product purity. This has the desirable effect of reducing the capital cost of the process equipment required for a given volumetric production rate. The present invention as described below and defined by the claims which follow addresses this need by reducing the required pressure equalization time in a cycle, thereby reducing the overall cycle time and increasing product recovery per unit of adsorbent used.
The invention is a pressure swing adsorption process for recovering a less strongly adsorbable component from a feed gas mixture comprising at least one less strongly adsorbable component and at least one more strongly adsorbable component, which process comprises performing sequential process steps in an adsorbent bed which include an adsorption step, two or more pressure equalization steps at decreasing pressure, a provide purge step, a blowdown step, a purge step, two or more pressure equalization steps at increasing pressure, and a final repressurization step, wherein the duration of each pressure equalization step is less than about 25 seconds and the adsorbent bed is one of at least four parallel adsorbent beds undergoing the sequential process steps in a cyclic manner.
In the process, four or more pressure equalization steps can be carried out at decreasing pressure and four or more pressure equalization steps can be carried out at increasing pressure in the adsorbent bed. Optionally, the provide purge step and the blowdown step can overlap such that gas is withdrawn from the product end of the bed for providing purge to another bed while additional blowdown gas is withdrawn from the other end of the bed. Optionally, the final pressure equalization step at decreasing pressure and the provide purge step can overlap such that gas is withdrawn from one end of the bed for providing purge to another bed while additional gas is withdrawn from the other end of the bed to pressurize yet another bed. Three beds can be on the purge step at any given time.
The adsorbent bed can be one of 12 more parallel adsorbent beds. The feed gas mixture can contain hydrogen, carbon monoxide, carbon dioxide, and one or more hydrocarbons containing one or more carbon atoms, wherein hydrogen is the less strongly adsorbable component. In this process, a series of two or more pressure equalization steps can be effected between an adsorbent bed and other adsorbent beds, and the final differential pressure in one of the pressure equalization steps can be greater than the final differential pressure in any earlier step in the series of two or more pressure equalization steps.
In one embodiment of the invention, the process uses 16 parallel adsorbent beds which undergo the sequential process steps in a cyclic manner. Four beds can be on the adsorption step at any given time. In addition, four pressure equalization steps can be carried out at decreasing pressure and four pressure equalization steps can be carried out at increasing pressure in each of the 16 adsorbent beds. Also, three beds can be on the purge step at any given time. Preferably, each bed on the purge step is purged exclusively by gas provided exclusively from another bed on the provide purge step. Optionally, an idle period can follow the purge step.
In another embodiment of the invention, 14 adsorbent beds are utilized which undergo the sequential steps in a cyclic manner. Three or four beds can be on the adsorption step at any given time and two or three beds can be on the purge step at any given time. Preferably, four or five pressure equalization steps can be carried out at decreasing pressure and four or five pressure equalization steps can be carried out at increasing pressure in each of the 14 adsorbent beds. Optionally, the provide purge step and the blowdown step can overlap such that gas is withdrawn from one end of the bed for providing purge to another bed while additional blowdown gas is withdrawn from the other end of the bed.
In yet another embodiment, 12 adsorbent beds are utilized which undergo the sequential steps in a cyclic manner. Two or three beds can be on the adsorption step at any given time. At least one of the pressure equalization steps at decreasing pressure and the provide purge step can overlap such that gas is withdrawn from the product end of the bed for providing purge to another bed while additional gas is withdrawn from the other end of the bed to pressurize yet another bed. Two beds can be on the purge step at a given time and three beds can be on the purge step at another given time. Optionally, the provide purge step and the blowdown step can overlap such that gas is withdrawn from the product end of the bed for providing purge to another bed while additional blowdown gas is withdrawn from the other end of the bed.
The invention includes a pressure swing adsorption system for operation in a pressure swing adsorption process, wherein the system comprises 16 parallel adsorbent beds which are manifolded such that three beds can be on a purge step at any given time and each bed on the purge step can be purged exclusively by gas provided exclusively from another bed on a provide purge step. The 16 parallel adsorbent beds can be manifolded into four groups of four beds each such that each bed in any given group can provide purge gas exclusively to another bed in the group and can receive purge gas exclusively from yet another bed in the group. The 16 parallel adsorbent beds can be manifolded such that a group of four beds can be isolated from the other 12 beds and the other 12 beds can be operated in a cycle with two beds on feed at any given time, 2 beds on purge at any given time, and two pressure equalizations.
Another embodiment of the invention includes a pressure swing adsorption system for operation in a pressure swing adsorption process, wherein the system comprises 12 parallel adsorbent beds which are manifolded such that two beds can be on a purge step at any given time and each bed on the purge step can be purged exclusively by gas provided exclusively from another bed on a provide purge step. The 12 parallel adsorbent beds can be manifolded into three groups of four beds each such that each bed in a group can provide purge gas exclusively to another bed in the group and can receive purge gas exclusively from yet another bed in the group. The 12 parallel adsorbent beds can be manifolded such that the given group of 4 beds can be isolated from the other 8 beds, wherein the other 8 beds can be operated in a cycle with one or two beds on feed at any given time, one or two beds on purge at any given time, and three pressure equalizations.